Analog phase-locked loop circuits typically can operate with minimal jitter across just a limited frequency range. Accordingly, it is usually necessary to prepare a number of different circuit designs in order to accommodate different applications requiring operation in different frequency ranges. This is inefficient in terms of design time and product cost, particularly where the phase-locked loop is implemented in an integrated circuit.
A typical analog phase-locked loop circuit includes a phase detector which determines the phase relationship between a reference signal and the output of the phase-locked loop, a charge pump responsive to the phase detector for setting a current or voltage which represents the phase relationship, a low pass filter which has a single cutoff frequency and is responsive to the current or voltage from the charge pump, and a voltage controlled oscillator which generates the output signal from the phase-locked loop circuit and which varies the frequency of the output signal in response to a control voltage from the low pass filter. A voltage regulator may optionally be present in order to provide a regulated supply voltage to the voltage controlled oscillator.
Existing voltage controlled oscillators for analog phase-locked loops have a single control voltage input that varies the frequency of the voltage controlled oscillator. If a wide frequency range is desired, then the voltage controlled oscillator is designed so that the single input has a large gain, expressed in Hertz/volt. However, the larger the gain, the more the jitter induced in the analog phase-locked loop as the charge pump increases and decreases the voltage on the loop filter capacitor.
Where such an analog phase-locked loop is implemented in an integrated circuit, at least three external connection pins of the integrated circuit are dedicated to the analog phase-locked loop circuit. In particular, these three pins provide the analog phase-locked loop circuit with a regulated supply voltage and an associated ground, along with a connection to an external filter circuit to allow limited variation of the frequency characteristics for different applications. It is, of course, desirable in the design of any integrated circuit to reduce the number of external connection pins needed for any circuit section within the integrated circuit. Other important design considerations for integrated circuits are reduction in the area needed to implement a particular circuit function, and reduction in the amount of power consumed by the circuitry implementing a particular function.